Ubuntu Light is a tailored version of Ubuntu designed primarily for use in a dual-boot environment with netbook computers and other devices. Ubuntu Light is designed to be another "instant-on" Linux operating system like SplashTop and others that have come about since that point like HyperSpace. Canonical is also looking at making an Ubuntu Light version for the desktop in the future too.
Running on Ubuntu Light is a new desktop interface they have dubbed "Unity", which they have designed based upon pictures of your desktop that Mark Shuttleworth requested last year. From the provided screenshot in Mark's blog post, Unity looks pretty interesting for a bare-bones netbook and they have more features on the way with the 10.10 edition of Ubuntu Light like support for many more applications and better file management.
Read Mark's blog post for all of the details. There is also an Ubuntu Light product page where they are offering up this newest member of the Ubuntu family to device vendors and other OEMs. According to that page, Ubuntu Light is booting to the web in seven seconds on a Dell Mini 10v.